Allow for init to run on an existing directory.

Instead of failing on an existing directory the init now uses a temp directory and then transfers the files over to the pod root optionally force replacing files.
Merge branch 'master' of github.com:grow/pygrow into feature-init-existing-dir
Merge branch 'master' into feature-init-existing-dir
Merge pull request #21 from Zoramite/feature-init-existing-dir

Allow for init to run on an existing directory.